As always, you pay a premium for the extra memory. The problem is at this level since we're using such high-end GDDR5 modules that price premium is quite major. $529.99 for what is essentially an overclocked HD 5870 is a lot of money and really, you're in the realm of GTX 480 money.
Having said that, the HD 5870 continues to be a more attractive option than the GTX 480 for many people due to quieter noise levels and less heat. At the same time, regular driver updates and the maturity has really established the HD 5870 as a fantastic card.
Really, though, I don't see a reason to buy a 2GB HD 5870 over a standard 1GB one. For many that's not going to matter, however. Really, if you want the extra memory you're better off buying a model like this which carries with it a really nice overclock, but more importantly, a really nice cooler.
The best thing Sapphire did was send us the Vapor-X version of the 2GB card for the simple reason that it's able to stand out against the competition thanks to the cooler and the overclock.
If you're after the extra memory and happy to spend the extra money that is associated with it, have a good look at the Sapphire Vapor-X model. While it does lack a little in the bundle department with no real extras being present, the hero of the whole setup is the card and it does a fantastic job of being a knight in shining armor.